Process of pretreating a pozzolanic material for increasing the pozzolanic properties of said material

ABSTRACT

The pozzolanic properties of a pozzolanic material, more particularly a fly ash, are improved by mixing the pozzolanic material with a calciumoxyde or calciumhydroxide containing material and water in such a way that at the time of associating or bringing together the abovementioned components the mixture has or immediately attains a temperature of more than 50° C., preferably at least 80° C., most preferably 85° to 100° C. 
     A mixture of pozzolanic material and calciumoxyde and/or calciumhydroxide may be preheated, subsequently, water being added to said mixture. 
     Water as used may be hot water and said water is preferably added as one mass at one moment. 
     The pozzolanic material obtained from the process of the invention is suitable as cement substitute, as material for manufacturing building articles and a binder for occluding one or more fillers to be introduced into a chemical process or one or more fillers presenting noxious properties.

This is a continuation of application Ser. No. 853,458 filed Apr. 18,1986, now abandoned.

BACKGROUND OF THE INVENTION

The invention relates to a process of increasing the pozzolanicproperties of a pozzolanic material, particularly fly ash fromcoal-fired units such as vessels of coal-fired power stations andwhirling bed vessels and similar types of fly ash in order to increasethe reactivity in relation to binding agents. By way of distinctionbinding agents can be divided into those reacting directly with fly ashsuch as calcium hydroxide containing substances and those reactingindirectly such as cements and unslaked limes.

Fly ashes are generally held to be pozzolanic i.e. in water they reactwith calcium hydroxide and subsequently harden.

Practice shows that such hardening proceeds very slowly and that thereexist quite a few fly ashes that do not or hardly harden at all althoughsuch fly ashes do absorb a large quantity of calcium hydroxide.

SUMMARY OF INVENTION

It is an object of the invention to provide a process of pretreating apozzolanic material, particularly fly ashes in such a way that inconjunction with calcium hydroxide containing substances and water,conceivably in the presence of other substances, this leads tohardenable mixtures attaining very high degrees of strength afterhardening.

According to the invention this aim is attained in that a process ofincreasing the pozzolanic properties of a pozzolanic materialparticularly a fly ash is characterized in that during the action ofwater upon a mixture of pozzolanic material and calcium hydroxide and/orcalciumhydroxide said mixture has or attains immediately a temperatureof more than 50° C.

Preferably the mixture has or immediately attains a temperaturecomprised between 80° and 100° C. more particularly a temperaturecomprised between 90° and 95° C.

It appears that by using a high temperature at the time of associatingor bringing together the three components of the process viz. apozzolanic material, calcium oxide and/or calcium hydroxide and waterpozzolanic materials, more particularly fly ashes which do not react oronly react very slowly are activated so that the affinity of thepozzolanic material, particularly fly ash, in relation to calciumhydroxide is effected in a very favourable manner.

The pretreated product as obtained according to the invention can beused for manufacturing building materials, making a binding agent forabsorbing and occluding noxious substances or for absorbing chemicalcompounds to be introduced into a process. The strengths of the hardenedbinder, the combination of pretreated pozzolanic material moreparticularly fly ash, calcium hydroxide containing substance, water andconceivably other substances can be brought to a very high value byproper composition.

Use of the pretreated pozzolanic material, more particularly fly ash, incement also provides better results as with non-pretreated fly ash. Forthe use in cement such fly ash has only to be subjected to apretreatment, and subsequently a proper drying so that the fly ash isobtained in powder form suitably for mixing with cement. The strengthsof hardened mixtures of pretreated fly ash, calcium hydroxide containingsubstance and water is of the same level as obtained by hardeningmixtures of grinded granulated blast furnace slag material and calciumhydroxide containing substance and water.

Applying the process according to the invention for manufacturingbuilding material wherein fly ash in bound directly boils down to thefly ash being pretreated during the mixing of ash, lime and water whichis preferably added in the form of hot water and possibly anothersubstance or other substances whereupon the mass is made into a granularmaterial or into the shape of the building material desired in order tobe subsequently hardened.

Moreover the pretreatment method according to the invention offers thepossibility of being able to combine fly ash better with cement andaffords an extended range of the applicability of fly ashes.

The binding agent, the mixture of pretreated fly ash, water and lime canalso be used for agglomerating a waste product of blast furnaceoperations such a oxysludge and for pelletizing or briquetting phosphateore or bauxite-containing substances, to which fly ash may be consideredto belong as well. Furthermore the binding agent is also quite suitablefor the treatment of other substances.

Variation in the amount of calcium hydroxide to be incorporated in themixture of pretreated fly ash, calcium hydroxide and water does notchange the strengths of the hardened binder. Generally speakingdepending upon the hardening conditions, the amount of calcium hydroxidein the binder may vary from some % by weight to about 20% by weight.Many compounds may be incorporated in the binder. Some of thesesubstances are gypsum of the phosphoric acid preparation or from theflue gas desulfuration, harbour sludge etc.

Soil containing impurities, waste from sewage purification, phosphateore, iron ore, finally divided metal and metal compounds, carbon black,cokes or combinations of some of these substances, oxycalcium hydroxidesludge or oxysludge.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example I

A mixture of 93 kg cold fly ash from a coal-fired power plant, 7 kgslaked calcium hydroxide mainly consistly of calcuim hydroxide is mixedwith 20 kg of water at a temperature of 20° C. After mixing the mixtureis granulated by adding some additional water.

The granules as obtained are cured in a steam atmosphere of 95° C. for 8hours.

The same mixture as mentioned hereinbefore is prepared but the way ofmixing is different.

First of all the fly ash is mixed dry with the slaked lime and the wholeis brought to a temperature of about 92° C. in a double-walled mixer.The water as used has a temperature of 95° C. The hot water and theheated mixture are then associated and mixed. Hardening is executed inthe same way as hereinbefore in a steam atmosphere of 95° C. for 8hours.

After hardening the grain strengths of the grains from the sievefraction 10-13 mm is in the first case 25 kg while by applying a heatedmixture and heated water the grainstrength amounts to 70 kg.

The grain strength is determined by placing a grain between two platesand subsequently exerting a pressure upon one of the plates. The pointload pressure causing break of the grain is determined.

Example II

Two mixtures are prepared but instead of 7 kg slaked lime, 5.4 kg ofunslaked lime consisting for 92% of calcium hydroxide is used. Thetemperature of the mixture does not rise above 65° C.

In a comparative experiment the components are heated as described inexample I so that during mixing from the first contact of the fly ashwith the water the temperature raised above 80° C. Due to the reactionheat emitted by the reaction of unslaked lime and water the temperaturerises very rapidly and calcium hydroxide dissolves so that the reactionsforming the basis for the later strength in the product are able toproceed above 90° C. The grains obtained from the non preheated mixturepresent a grain strength of 18 kg while the grains obtained from thepreheated mixture present a strength of 88 kg.

Example III

Instead of 7 kg slaked lime as in example 1, 10 kg of Portland cement isused. The effect of the pretreatment occurs but is relatively smaller.

Cold mixing provides grains with a grain strength of 16 kg and hotmixing provides a grainstrength of 22 kg.

The amount of cement in the mixture was 9%.

As cement after contact with water only provides some calcium hydroxideafter some hours this experiment was repeated in combination with slakedlime.

For comparative purposes two mixtures are prepared consisting of:

Fly ash--42.5% by weight.

Coal-ash--28.3% by weight.

Slaked lime--3.0% by weight.

Cement p.c.a.--3.4% by weight.

Water--22.8% by weight.

The grainstrength of the grains of the sieve fraction 10-13 mm has avalue of 18 kg when cold mixing is used, and a mixture obtained by hotmixing provides a grain strength of 40 kg.

Example IV

Example III is repeated but 1% by weight of a mixture of gypsum andcalciumsulfite is added to the mixture.

The grainstrenght is 22 kg when cold mixing is used and in the case ofhot mixing (a temperature of the mixture comprised between 90° C. and100° C. immediately as water contacts the mixture) 58 kg.

Example V

In a mixer the temperature of the fly ash is increased by introducingsteam. After having reached a sufficiently high temperature hot water of98° C. is added to the mixture of ash and condensed water (4%). Theamount of unslaked lime amounted to 5.5% by weight based on the weightof the dry ash. The temperature during the contact of the water with thedifferent components was 91° C. as some steam escaped from the mixer.After mixing the mixture is stored for some time in a container andsubsequently granulated. A part of the granules is stored in the air andanother part is hardened by means of a heat treatment by steam asdescribed in example I.

The granules obtained from a mixture prepared by mixing at increasedtemperature have a grain strength (sieve-fraction from 10-13 mm) of 79kg.

The granules hardened under air present a strength of 54 kg after twoweeks storage under air which is already sufficient for manyapplications in concrete.

Here the increase of strength depends on the time.

Already after one week a grain strength of 36 kg is obtained.

Example VI

The temperature of the mixture in the mixer is not brought in the rightrange so that the temperature is below 50° C. Hardened granules onlyprovide a grain strength of 30 kg whilst the granules stored in the airpresent a grain strength of 14 kg after one year.

Example VII

A mixture obtained by hot mixing of 9,5 kg fly ash and 5 kg slaked limeis dried and stored for some time. After grinding the material is mixedwith Portland cement in a ratio of 1:1. A part of this mixture issubsequently mixed with sand in the ratio 30% mixture and 70% sand. Fromthe mixture a cube of 7×7×7 cm is pressed. This cube is hardened.

A second cube is manufactured from a mixture of 70% sand and 30% lastfurnace cement which cube is hardened in the same way as the first cube.

The strength of the cube obtained from fly ash amounts to 380 kg/cm²whilst the cube obtained from blast furnace cement has a strength of 360kg/cm².

This result is very remarkable as literature describes many experimentswith fly ashes which only show a very small increase of strength byreplacing small amounts of cement by this mixture or even descreases ofstrength.

Example VIII

Phosphoric acid waste gypsum is mixed with fly ash in the ratio of e.g.1:1. By blowing steam through this mixture in a mixer the temperature isincreased to about 94° C. Subsequently calcium hydroxide is added andthe whole amount of hot water is rapidly introduced into the mixer byinstantaneously opening the valve of a container containing said water.Subsequently the material is granulated, the grain strength of thegranules of the sieve fraction 10-13 mm is 80 kg.

Cold mixing of such a mixture provides granules having a strength ofonly 40 kg.

What is claimed is:
 1. In a process of increasing the pozzolanicproperties of a pozzolanic material, including fly ash, the steps ofmixing pozzolanic material, calcium oxide and/or calcium hydroxide andwater, at least one of said pozzolanic material, calcium oxide and/orcalcium hydroxide and water being pretreated in such a manner that atthe time of adding the water to a mixture of pozzolanic material andcalcium oxide and/or calcium hydroxide the resulting mixture immediatelyattains a temperature of 80-100 deg. C.
 2. In a process according toclaim 1, wherein the total amount of water is added to a mixture of flyash and calcium oxide and/or calcium hydroxide as one mass at one time.3. In a process according to claim 1, wherein a mixture of heated flyash and calcium oxide and/or calcium hydroxide is mixed with water. 4.In a process according to claim 1, wherein pozzolanic material, calciumoxide and/or calcium hydroxide and preheated water are mixed.
 5. In aprocess according to claim 1, wherein the resulting mixture is used as asubstitute for cement, such that the resulting mixture has orimmediately thereafter attains a temperature of 80°-100° C.
 6. In aprocess according to claim 1, wherein the resulting mixture ofpozzolanic material, water and calcium hydroxide after having attained atemperature of 80°-100° C. is molded into an article being cured.
 7. Ina process according to claim 1, wherein the resulting mixture is mixedas binding agent with a filler, consisting of phosphoric acid wastegypsum and the mixture with said filler is molded into articles whichare cured.